This invention relates to a composite piston for internal combustion engines, comprising an upper part, which consists of ferrous material and is joined by a centering screw-threaded ring to a lower part of the piston.
In a piston engine, the combustion of the fuel which has been supplied is initiated in an Otto engine by an electric spark and in a diesel engine by the injection of the fuel into the air which has been highly compressed in the combustion chamber during the compression stroke. The energy which is originally present as chemical energy is converted by the combustion almost entirely into thermal and pressure energy. Thereafter, about one-third of the energy is is converted by means of the piston and the power train into mechanical energy. It is known that owing to thermodynamic effects which are not explained here in detail, the remaining two-thirds of the supplied energy are dissipated in approximately equal parts in the coolant and the exhaust gases.
There is a considerable desire in the art to minimize that part of the energy content of the combustion gases which is transferred to the coolant. This could be accomplished by the provision of a piston head consisting of a material which combines a low thermal conductivity with high heat resistance and high mechanical strength. The materials previously for this purpose do not meet these requirements. For instance, ceramic materials have no optimum mechanical strength properties and high-temperature steels have a relatively high thermal conductivity.